The invention relates to a novel process for preparing 2,7-dimethyl-2,4,6-octatrienal monoacetals (also referred to hereinafter as C10-dialdehyde monoacetals) of the general formula I, 
in which the substituents R1 and R2 may, independently of one another, be C1-C8-alkyl or, together with the oxygen atoms and the carbon atoms to which they are bonded, form a 1,3-dioxolane or 1,3-dioxane ring of the following structures. 
Monoacetals of this structure are required according to Helv. Chim. Acta 1981, 64 (7), 2469 for the selective synthesis of C40-carotenoids with a nonsymmetrical structure.
Protection of one of the two carbonyl groups in the central C10 building block (9) in the following reaction scheme makes it possible to carry out the Wittig reactions with the two C15 phosphonium salts (2) and (5) very selectively in succession and thus obtain a carotenoid (6) which is free of the products (7) and (8) with symmetrical structures, whose formation is unavoidable on use of an unprotected C10-dialdehyde (9). 
Of particular interest from the group of carotenoids which have a nonsymmetrical structure with different end groups A and B are lutein (10) and meso-zeaxanthin (11) because these carotenoids protect inter alia the human eye from blindness resulting from age-related macular degeneration [Exp. Eye Res. 1997, 64 (2), 211-218; GB 2301775 (1996)]. 
In order to obtain meso-zeaxanthin (11) which is free of R,R-zeaxanthin (12) and S,S-zeaxanthin (13) it is necessary, for example, for the two Wittig reactions of the central C10 building block with the Rxe2x80x94C15 phosphonium salt (14) to give a C25 intermediate (15), and its reaction with the Sxe2x80x94C15 phosphonium salt (16) to take place completely selectively in succession. 
These requirements also apply analogously to the synthesis of lutein.
The selectivity of the two Wittig reactions which is necessary for synthesizing a pure product is ensured only if a C10-dialdehyde in which one carbonyl group is in protected form as acetal is employed.
Various syntheses of C10-dialdehyde monoacetals have been described in the literature:
Thus, for example, the dimethyl acetal (17) can be obtained by p-toluenesulfonic acid-catalyzed acetalyzation of the dialdehyde (9) [Helv. Chim. Acta 1981, 64 (7), 2469]. The process includes a complicated low-temperature crystallization, and the product can be obtained only in inadequate yield. 
Another synthetic process relates to the formation of the aldehyde function by reduction of the corresponding nitrile (18) [Pure and Appl. Chem. 1994, 66 (5), 963; Recl. Trav. Chim. Pays-Bas 1994, 113, 552]. This reduction is effected with diisobutylaluminum hydride at xe2x88x9270xc2x0 C. The complete synthesis, which is described only on the mmol scale, has the aim of introducing 13C isotopes and has no industrial significance.
The synthesis described in the literature of the C10-dialdehyde mononeopentyl glycolacetal (19) is based on the selective cleavage of the corresponding bisneopentyl glycolacetal (20) to the monoacetal by brief contact with HCl. This synthesis is likewise described only on the mmol scale. Because the selectivity is poor it was possible to isolate pure monoacetal (19) in a yield of only 37% only after elaborate purification by crystallization twice (Helv. Chim. Acta 1981, 64 (7), 2471) This synthesis is also unsuitable for industrial implementation. 
It is an object of the present invention to provide a process for preparing C10-dialdehyde monoacetals which can be implemented on the industrial scale and has maximum flexibility in relation of the protective group.
We have found that this object is achieved by a process for preparing 2,7-dimethyl-2,4,6-octatrienal monoacetals of the general formula I, 
in which the substituents R1 and R2 may, independently of one another, be C1-C8-alkyl or, together with the oxygen atoms and the carbon atoms to which they are bonded, form a 1,3-dioxolane or 1,3-dioxane ring of the following structures 
in which R3 and R4, and R5 may each, independently of one another, be hydrogen or C1-C4-alkyl,
which comprises
a) condensing an ester phosphonium salt of the general formula II or an ester phosphonate of the general formula III, 
in which the substituents have, independently of one another, the following meaning:
R6 C1-C8-alkyl;
R7 aryl;
Xxe2x88x92 an anion equivalent of an inorganic or organic acid;
R8 and R9 
C1-C8-alkyl;
with an aldehyde of the formula IV 
in a Wittig or Wittig-Horner reaction to give an acetal ester of the general formula V, 
in which the substituents R1, R2 and R6 in the compounds IV and V have the abovementioned meaning,
b) reducing the ester of the formula V to an acetal alcohol of the general formula VI, 
and
c) oxidizing the alcohol to 2,7-dimethyl-2,4,6-octatrienal monoacetals of the general formula I.
It was surprisingly possible to achieve this object in a simple manner, starting from C5 building blocks which have a fixed place in polyene synthesis and are easily obtainable on the industrial scale (cf. Carotenoids, Vol. 2, xe2x80x9cSynthesisxe2x80x9d, pp. 115 ff.; Birkhauser Verlag 1996).
In the case of open-chain acetals, alkyl radicals which may be mentioned for R1 and R2 are linear or branched C1-C8-alkyl chains, e.g. methyl, ethyl, n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl, n-heptyl and n-octyl.
Preferred alkyl radicals for R1 and R2 are methyl, ethyl, n-propyl and 1-methylethyl, particularly preferably methyl and ethyl.
Alkyl radicals which may be mentioned for R3 to R5 are linear or branched C1-C4-alkyl chains, e.g. methyl, ethyl, n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl and 1,1-dimethylethyl.
Preferred radicals for R3 to R5 are hydrogen and methyl.
Alkyl radicals which may be mentioned for R6, R8 and R9 are linear or branched C1-C8-alkyl chains, e.g. methyl, ethyl, n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl, n-heptyl and n-octyl.
Preferred alkyl radicals for R6, R8 and R9 are methyl, ethyl, n-propyl and 1-methylethyl, particularly preferably methyl and ethyl, very particularly preferably ethyl.
The term aryl for R7 refers to conventional aryl radicals occurring in phosphines and phosphonium salts, such as phenyl, tolyl, naphthyl, optionally substituted in each case, preferably phenyl.
The radical Xxe2x88x92 represents one anion equivalent of an inorganic or organic acid, preferably a strong inorganic or organic acid.
The term strong acid encompasses hydrohalic acids (especially hydrochloric acid and hydrobromic acid), sulfuric acid, phosphoric acid, sulfonic acids and other inorganic or organic acids with a comparable degree of dissociation. Strong organic acids also mean in this connection C1-C6-alkanoic acids such as formic acid, acetic acid, propionic acid, butyric acid and caproic acid.
Particularly preferred anions which should be mentioned are acids selected from the group consisting of hydrochloric acid, hydrobromic acid, sulfuric acid, phosphoric acid, formic acid, acetic acid and sulfonic acids, very particularly preferred Clxe2x88x92, Brxe2x88x92, CnH2n+1xe2x80x94SO3xe2x88x92 (with n=1-4), Phxe2x80x94SO3xe2x88x92, pxe2x80x94Tolxe2x80x94SO3xe2x88x92 or CF3xe2x80x94SO3xe2x88x92.
The first step a) in the process of the invention comprises the olefination reaction of a C5-ester phosphonium salt of the general formula II or of a C5-ester phosphonate of the general formula III with a C5-acetal aldehyde of the general formula IV 
in which the substituents have the meaning mentioned at the outset.
A preferred embodiment of process step a) comprises using an ester phosphonate of the general formula III in which the substituents R8 and R9 are, independently of one another, C1-C3-alkyl, particularly preferably methyl or ethyl, very particularly preferably ethyl.
A likewise preferred embodiment of process step a) comprises using as aldehyde a compound of the formula IVa 
in which the substituents R3 and R4 are, independently of one another, hydrogen and/or methyl, preferably in each case jointly hydrogen or methyl, particularly preferably jointly methyl.
The Wittig condensation of the phosphonium salt II with the aldehyde IV or the Wittig-Horner condensation of the phosphonate III with IV to give a C10-acetal ester of the formula V is carried out under the conditions typical for these reactions (see Carotenoids, Vol. 2, xe2x80x9cSynthesisxe2x80x9d, pp. 79 ff.; Birkhauser Verlag, 1996, and literature cited therein).
Condensation of II with IV can be carried out, for example, in an inert organic solvent, e.g. in open-chain or cyclic ethers such as diethyl ether, diisopropyl ether, methyl tert-butyl ether, 1,4-dioxane or THF, in halogenated hydrocarbons such as dichloromethane, chloroform, in aromatic hydrocarbons such as toluene, xylene or benzene, or in polar solvents such as dimethylformamide, dimethyl sulfoxide or acetonitrile. Preferred solvents are toluene, THF and DMSO or mixtures thereof.
It is possible to use as base all the bases customary for Wittig condensations, e.g. alkali metal hydroxides such as sodium hydroxide, potassium hydroxide or lithium hydroxide; alkali metal hydrides such as sodium hydride or potassium hydride.
Other suitable bases are organolithium compounds such as, for example, n-butyllithium, tert-butyllithium, phenyllithium or alkali metal amides such as lithium, potassium or sodium amide, lithium diisopropylamide or else alkali metal hexamethyldisilazides. The base preferably employed for the Wittig reaction of the invention is sodium or potassium hexamethyldisilazide, or potassium or sodium amide.
The amount of base employed is usually in the range from 0.8 to 5 mol, preferably 1 to 3 mol, per mole of the phosphonium salt II employed.
If Xxe2x88x92 is a halide anion, it is also possible and advantageous to employ oxiranes as latent bases (see Chem. Ber. 1974, 107, 2050).
The bases preferably used for this Wittig reaction are solutions of alkali metal alcoholates in the corresponding alcohol or oxiranes, especially 1,2-epoxybutane, without additional solvent or in a mixture with one of the abovementioned solvents or a lower alkanol.
The conditions used for the Wittig-Horner reaction of III with IV are likewise those typical for this reaction.
In this case too, one of the abovementioned inert organic solvents is used, and a solution of an alkali metal alcoholate in the corresponding alkanol is preferably employed as base. However, in the case of the Wittig-Horner reaction, it is also possible to use the bases additionally mentioned above for the Wittig reaction with the exception of the oxiranes.
In the next steps of the process, the ester V is converted into the corresponding aldehyde.
Various methods are described in the literature for converting esters directly into aldehydes. However, these methods involve complex hydrides which are not available industrially, such as, for example, piperazinylaluminum hydrides (Chem. Lett. 1975, 215) or reagents which are difficult to handle industrially, such as, for example, diisobutylaluminum hydride, which additionally requires low reaction temperatures (xe2x88x9270xc2x0 C.).
However, an industrial process must comply with the following requirements:
the reaction must take place completely and with maximal selectivity in order to avoid elaborate and costly separation operations
extreme reaction conditions, especially low temperatures, must be avoided
the reagent must be available industrially in relatively large quantities and be easy to handle from the viewpoint of safety.
On the basis of these requirements, the ester group is converted in the process of the invention in a two-stage procedure [stages b) and c)] into the aldehyde function.
In step b), the ester of the formula V is initially reduced to the corresponding alcohol of the formula VI. 
It is possible in principle to employ for this step all hydride reagents described for reducing esters to alcohols, for example alkali metal borohydrides or alkali metal aluminum hydrides.
A preferred embodiment of process step b) comprises carrying out the reduction of the ester function with a sodium aluminum hydride compound, particularly preferably with sodium dihydrobis(2-methoxyethoxy)aluminate.
In accordance with the abovementioned requirements for an industrial process, the commercially available concentrated toluene solution of sodium dihydridobis(2-methoxyethoxy)aluminate (xe2x80x9cVitride(copyright)xe2x80x9d) is particularly advantageous. This reagent is not pyrophoric, not sensitive to oxygen (GIT Fachz. Lab. 9/96, 914) and, as a liquid, is considerably easier to handle in an industrial process than are solid complex hydrides such as, for example, lithium aluminum hydride.
The reaction is preferably carried out in such a way that the ester of the formula V is introduced into a solvent which is inert toward hydride reagents, such as, for example, toluene, open-chain or cyclic ethers, glycol ethers or a mixture of the solvents, and the reducing agent is metered in at a temperature in the range from xe2x88x9220xc2x0 C. to 30xc2x0 C., preferably from xe2x88x9210xc2x0 C. to 10xc2x0 C., particularly preferably from xe2x88x925xc2x0 C. to 0xc2x0 C.
It is usual to employ at least 2 equivalents of hydride per equivalent of ester, i.e. at least 0.5 mol of lithium aluminum hydride/mol of ester or 1.0 mol of Vitride/mol of ester. However, in order to achieve complete conversion, it is advantageous to employ a certain excess of reducing agent. This excess is in the range from 10 to 50 mol %, preferably 20 to 30 mol %.
In the preferred embodiment of the process of the invention, the C10-acetal ester V is reduced with the toluene solution of Vitride to the C10-acetal alcohol VI. An aqueous workup results in a virtually quantitative yield of a crude product which can be employed directly, without purification, in the next stage c).
Oxidation of a C10-acetal alcohol of the formula VIa to the corresponding aldehyde has not to date been described in the literature. 
The closest prior art is the oxidation of a C10-dimethyl acetal alcohol VII to the corresponding aldehyde. 
As described in Helv. Chim. Acta 1966, 49, 369, this reaction is carried out with a large excess of manganese dioxide in a reaction time of 60 hours. The disadvantage of this process is, apart from the extremely long reaction time and the small yield, the use of a multimolar excess of oxidizing agent together with a not inconsiderable complexity in the removal of solids and the disposal of the manganese salts.
The oxidation processes mentioned in DE-A-3705785, DE-A-4440286, DE-A-4440287 and EP-A-0 718 283 for converting polyene alcohols into polyene aldehydes are suitable, inter alia, for the oxidation of VI to I in step c) of the process of the invention. However, catalytic methods will be preferred from the abovementioned economic, ecological and technical viewpoints of the process. Catalysts which can be employed for this purpose are, inter alia, ruthenium compounds such as tetrapropylammonium perruthenate, tris(triphenylphosphine)ruthenium(II) chloride or 1,5-cyclooctadieneruthenium(II) chloride in amounts of from 2 to 4 mol % in the presence of at least stoichiometric amounts of 4-methylmorpholine N-oxide as cooxidant (see J. Chem. Soc. Chem. Commun. 1987, 1625).
Catalytic oxidation of VI to I preferably takes place with a mixture comprising 2,2,6,6-tetramethylpiperidin-1-oxyl/copper(I) chloride/dimethylformamide/oxygen or with a mixture comprising 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl/copper(I) chloride/dimethylformamide/oxygen in dimethylformamide as solvent. Further details of the oxidation are to be found, inter alia, in DE-A-3705785 and EP-A-0 718 283.
DE-A-3705785 and EP-A-0 718 283 describe oxidations of allyl alcohols which are substituted by methyl in the position xcex2 to the alcohol group, that is to say display no steric hindrance. It was therefore unexpected and surprising that the application of this reaction to allyl alcohols of the formula VI with methyl substituents in the a position provides, in complete and smooth conversion, the corresponding xcex1,xcex2-unsaturated aldehyde.
The process of the invention achieves the object described at the outset by a process which
a) starts from precursors which are easily obtainable industrially,
b) leads to the desired product in high overall yield in an only three-stage synthesis with simple process steps without
c) the need to purify intermediates by elaborate process steps.
The invention also relates to acetal esters of the general formula Va 
in which the substituents have, independently of one another, the following meaning:
R6 C1-C8-alkyl;
R1 and R2 
C1-C8-alkyl or together with the oxygen atoms and the carbon atoms to which they are bonded a 1,3-dioxolane or 1,3-dioxane ring of the following structures, 
in which R3 and R4, and R5 may each, independently of one another, be hydrogen or C1-C4-alkyl, where R1 and R2 are not methyl when R6 is methyl.
Preferred acetal esters have the formula Vb 
in which R6 is C1-C4-alkyl, and the substituents R3 and R4 may jointly be either hydrogen or methyl.
The invention additionally relates to acetal alcohols of the general formula VI 
in which the substituents have, independently of one another, the following meaning:
R1 and R2 
C1-C8-alkyl or together with the oxygen atoms and the carbon atoms to which they are bonded a 1,3-dioxolane or 1,3-dioxane ring of the following structures, 
in which R3 and R4, and R5 may each, independently of one another, be hydrogen or C1-C4-alkyl.
Preferred acetal alcohols have the formula VIa 
in which the substituents R3 and R4 may jointly be either hydrogen or methyl.
Concerning the definition of the substituents R1 to R6xe2x80x94in the general and preferred embodimentxe2x80x94reference may be made to the explanations given at the outset.
The process of the invention is to be illustrated in detail by the following example.